This is not my comment but an anonymously suggested and rejected edit to John Rennie's answer: Any application of AdS/CFT to a practical system in 3 space dimensions would require that the system is goverend by a quantum gauge field theory in (3+1) dimensions that has the following three properties: 1. Maximal supersymmetry. 2. Conformal invariance 3. Large number of colors Since no such system has been found so far to exist in nature, one must assume some reasonable approximations in applying AdS/CFT to a system. Quark gluon plasma being produced by LHC fits this criterion.
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DilatonFeb 23 '13 at 23:52

2 Answers
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According to the Backreaction blog there is evidence from heavy ion collisions that contradicts Ads/CFT. However even is this is true (I have no way of judging) all it contradicts is the application of Ads/CFT to heavy ion collisions, and everyone knows it could at best be an approximation because heavy ion collisions aren't supersymmetric.

At the risk of stepping beyond my (meagre) knowledge of the area, at the moment there can't be any experimental tests because the CFT bit of Ads/CFT has to be supersymmetric and there are no know supersymmetric physical systems.

Response to comments

As various comments have pointed out, my second paragraph is unnecessarily pessimistic. Ads/CFT has been applied in many areas where it is thought to be a good approximation if not an exact description. Also see Condensed matter application of AdS/CFT for more on this subject.

I'm sorry that I have to downvote in this case, because this is now wrong. There are tons of applications of AdS/CFT in different fields such as condensed matter, QCD, fluid dynamics, where it successfully helped theoretically explain things measured that could not be addressed by other explanations.
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DilatonJan 11 '13 at 11:06

One of the latest applications, where it is used to make predictions of new effects (in the context of the stability of the QGP) which could be measured in the future is described here for example.
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DilatonJan 11 '13 at 11:06

Fair enough, and I should have remembered that article since it's the same blog as the article I linked. If no-one upvotes my answer (i.e. no-one finds it helpful) I'll delete it.
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John RennieJan 11 '13 at 11:09

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The CFT bit of AdS/CFT doesn't have to be supersymmetric, but in practice people mainly study SUSY CFT's in this context. This is especially true since little non-SUSY interacting CFT's are known when D > 2. In a recent paper (1212.3616), general CFT's in D > 2 are claimed/proven to be dual to AdS theories that are local at superhorizon scales.
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VibertJan 11 '13 at 11:32

@JohnRennie could you not edit your answer and give for example a summary of the latest Backreaction article about the topic or some other nice application of the AdS/CFT business? Your answer reads as being completely dismissive about the usefulness of these considerations and this is from a physics and scientific point of view really wrong and not justified. Reading your answer as it is at present, gives not the right picture about the actual state of the art concerning AdS/CFT.
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DilatonJan 12 '13 at 17:21

An important result of AdS/CFT is the calculation of a lower bound for the ratio of viscosity and entropy density of a relativistic fluid,
$\frac{\eta}{s}=\frac{1}{4\pi}\frac{\hbar}{k_B}.$
This is consistent with the outcome of experiments at the RHIC, in which the role of the fluid is played by the quark gluon plasma.